Nitrous oxide production



May 5, 1970 s. RAVYIV ETAL NITROUS OXIDE PRODUCTION Filed Aug. 15, 1967Inventors fin United States Patent 3,510,412 NITROUS OXIDE PRODUCTIONSzmuel Raviv and Shimon Malkiely, Beersheva, Israel, assignors to TheState of Israel Ministry of Defence, Tel-Aviv, Israel Filed Aug. 15,1967, Ser. No. 660,698 Claims priority, application Israel, Sept. 5,1966,

26,453 Int. (:1. B01k 1/00 US. Cl. 204-103 3 Claims ABSTRACT OF THEDISCLOSURE The present invention concerns the manufacture of nitrousoxide-N O. Nitrous oxide serves, among others, as rocket fuel and is inhigh demand. An efficient and economic method for its production istherefore an important industrial requirement.

A variety of electro-negative metals and metal alloys are known that arepassivated by treatment with nitric acid and are normally either notdissolved at all or dissolved onl very slowly by this acid. Examples ofsuch metals and metal alloys are various kinds of stainless steel,Inconel (trade name, alloy containing approximate ly 78% Ni, 15% Cr and7% Fe), aluminium, chromium, nickel, uranium, titanium, zirconium, andtheir alloys. All electro-negative metals and metal alloys which arecharacterized by a passivation upon treatment with nitric acid will bereferred to hereinafter for short as metals of the kind specified.

It is also known that the resistance of metals of the kind specified tocorrosive acidic attack can be increased by so-called cathodicprotection. Such a protection consists in connecting the metal ascathode into an electrolytic circuit and the rate of corrosion of metalsof the kind specified so treated, that is the rate of their attack byacid, is reduced considerably.

It has surprisingly been discovered that if a metal of the kindspecified is inserted into nitric acid and simultaneously therewithconnected as cathode into an electrolytic circuit, and the currentdensity on the cathode as well as the acid concentration and temperatureare suitably selected the metal is dissolved. This discovery forms thesubject of Israel patent application No. 26,224 filed by the sameapplicants on July, 26, 1966.

Such a cathodic dissolution of metals of the kind specified is dependenton three operational parameters namely, the current density on thecathode, the nitric acid concentration and the temperature. These threeparameters will be referred to hereinafter for short as dissolutionparameters.

For each metal of the kind specified there exist various combinations ofdissolution parameters each characterized by the fact that while it doesnot yield any appreciable dissolution of the metal, an increase of thevalue of any of the parameters will cause dissolution. Any suchcombination of values for the dissolution parameters which does not yetproduce any appreciable dissolution of the metal while an increase ofany of the values causes dissolution, which combination, in other words,constitutes the demarcation between non-dissolution and dissolution,will be referred to hereinafter for short as threshold combination.

Obviously for each metal there exists a plurality of thresholdcombinations. Thus for a given current density on the cathode and agiven temperature a very definite acid concentration has to be selectedin order to result in a combination of current density, temperature andacid concentration that will constitute a threshold combination. Ifwhile keeping, for example, the temperature constant the current densityon the cathode is changing, the acid concentration must be changed aswell in order to obtain in this way another threshold combination.

In accordance with the present invention it has surprisingly been foundthat if a metal of the kind specified is inserted into an aqueous nitricacid solution and connected as cathode to a source of D.C., and acombination of the dissolution parameters is selected which does notexceed a threshold combination, N 0 is produced at the cathode.

Consequently the invention consists in a process for the production ofnitrous oxide comprising inserting into an aqueous nitric acid solutiontwo electrodes of which the one is of a metal of the kind specified andthe other of a conductive material that is inert to nitric acid underthe operational conditions, connecting the electrode of the metal of thekind specified as cathode and the inert electrode as anode to a sourceof DC. thereby to produce an electrolytic circuit, selecting values forthe dissolution parameters (as hereinbefore defined) which yield acombination that does not exceed a threshold combination (ashereinbefore defined), allowing an electric current to flow in saidcircuit and withdrawing a nitrous oxide containing gas mixture from nearthe cathode.

Examples of materials for the insert anode are various noble metals andtheir alloys, such as, for example, platinum.

It is believed that the production of nitrous oxide on the cathode isdue to a cyclic passivation and depassivation of the cathode. It isfurther believed that during the passivation oxygen and nitrogen areadsorbed in gaseous form on the surface of the cathode while during thede-passivation they are desorbed with the simultaneous formation ofnitrous oxide.

The method according to the invention is superior to all known methodsfor the production of nitrous oxide both as regards equipment andenergetic requirements, and is therefore considerably cheaper. Thismethod could not have been anticipated from anything known from theliterature.

As a rule there is obtained in accordance with the invention at thecathode a mixture containing in addition to nitrous oxide alsoelementary nitrogen, oxygen and hydrogen in varying proportionsdepending on the reaction conditions. For many purposes this mixture canbe used as is while for other purposes purified nitrogen su boxide maybe isolated from that mixture by methods known per se.

In the accompanying drawing there is illustrated, by way of exampleonly, a laboratory scale apparatus for carrying out the invention.

The apparatus here illustrated comprises two communi- :ating vessels 1and 2 serving, respectively, as anode and :athode compartments. Vessels1 and 2 communicate hrough a duct 3 fitted with control valves 4 and Sand inked to a funnel 6 serving for the supply of nitric acid.Compartment 1 is fitted with an inert electrode 7 serving 18 anode andcompartment 2 is fitted with an electrode 14 )f a metal of the kindspecified serving as cathode.

Compartment 1 is further fitted with an elongated ves- :el 8 comprisingan inlet valve 9 and an outlet valve 10. Vessel 8 serves for collectingthe gaseous anode prodicts.

Likewise, compartment 2 is fitted with two identical :longated vessels11a and 11b controlled, respectively, Jy inlet valves 12a and 12b andoutlet valves 13a and 13b. Vessels 11a and 11b serve for collecting thegaseous cath- )de products and are designed for alternating use so that-:he apparatus may operate continuously.

The invention is illustrated by the following Examples Without beinglimited thereto. In all these examples an apparatus of the kinddescribed above was used, havin a. platinum anode.

EXAMPLE 1 Composition of the gas in the cathode Jurrent density oncompartment ;he cathode in impJdcm. N20 N2 02 H2 Total EXAMPLE 2Concentration of HNO -3.3 N Temp-20 C. Cathode--stainless steel AISI-304Composition of the gas in the cathode Surrent density on compartment ;hecathode in impJdcm. N20 N1 Oz H2 Total EXAMPLE 3 Concentration of HN O 5N Temp.20 C. Cathode-stainless steel AISI-304 Composition of the gas inthe cathode Current density on compartment the cathode in ampJdcm. N 0N2 02 E: Total EXAMPLE 4 Concentration of HNO 2 N Temp.-30 C.Cathode-nuclear uranium (common designation for purified uranium).

Composition of the gas in the cathode 4 EXAMPLE 5 Concentration of HNO 2N Temp.30 C. Cathodenuclear uranium Composition of the gas in thecathode Current density on compartment the cathode in amp/(1cm; N20 N2Oz Hz Total Composition of the gas in the cathode compartment expressedin mL/farad Current density on the cathode in amp./dcm. N20 N2 Oz HzEXAMPLE 6 Concentration of HNO 2 N Temp.3 0 C.

Cathodetitanium 99.5% Current density on the cathode40 amp./dcm.

The composition of the gas obtained in the cathode compartment,expressed in volume percent was as follows:

Percent N 0 41.2 N 17.5 0 0.2 H 0 CO 42 EXAMPLE 7 Concentration of HNO.-2 N Temp.30 C. Cathode-Zircalloy 2 Current density on the cathode-40amp/dcm.

The composition of the gas obtained in the cathode compartment,expressed in volume percent was as follows:

Percent N 0 21 N 21.2 0 7.2 H 52 EXAMPLE 8 Concentration of HNO 2 NTemp.30 C.

Cathode-Zircalloy 2 Current density on the cathode-50 amp/dcm.

The composition of the gas obtained in the cathode compartment,expressed in volume percent was as follows:

. Percent N 0 30 N i 10 O 1 2 60 -electrodes of which one, the cathode,is of a metal selected from the group consisting of electro-negativemetals and metal alloys that are passivated by treatment with nitricacid, and the other, the anode, is a conductive material that is inertto nitric acid under the operational conditions, connecting saidelectrodes to a source of direct current to produce an electrolyticcircuit, allowing said current to flow under conditions which do notproduce any appreciable dissolution of the cathode, and isolatingnitrous oxide from the gas mixture released near the cathode.

2. Process according to claim 1, wherein the current density on thecathode is within the range of 1 to 150 amp/dcmF.

References Cited UNITED STATES PATENTS 4/1968 Romans 204147 DANIEL E.WYMAN, Primary Examiner 10 P. M. FRENCH, Assistant Examiner US. Cl. X.R.

